Whether IoC is reasonable or not does not depend on the number of classes in the project. It is still a useful pattern even if you have, like, 3 classes (component A using implementation B of the interface Ib). It's just if you have only few components, you may want to wire them manually in maybe 10 lines of code instead of dropping in Spring JARs.
So it's good when you need it, and it's unnecessary over-engineering for simple projects.